Why voltage divider for resistive sensors?
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Why voltage divider for resistive sensors?

by relaxing on Sun Dec 02, 2012 2:39 pm

Excuse my basic theory question, but...
I've read the Wikipedia article on voltage dividers and I'm pretty sure I understand what they do. But why do we build them into sensor circuits?

I've built Arduino circuits like...
5V------thermistor------Analog In
And they work.
So why is it better to do
5V------thermistor------Analog In------resistor-----GND
:?:

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Re: Why voltage divider for resistive sensors?

by adafruit_support_bill on Sun Dec 02, 2012 3:24 pm

This app note describes the pros & cons of various thermistor circuit configurations:
http://ww1.microchip.com/downloads/en/a ... 00685b.pdf
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Re: Why voltage divider for resistive sensors?

by relaxing on Sun Dec 02, 2012 4:16 pm

So the answer is linearization?
Is it fair to say if I'm unconcerned with linear response, I can still hook up my sensor to a GPIO without also connecting it to ground?

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Re: Why voltage divider for resistive sensors?

by adafruit_support_bill on Sun Dec 02, 2012 6:08 pm

If you have nothing connected to the input of an analog pin, it 'floats' and you get nothing but noise. If you connect a fixed pullup or pulldown resistor of sufficient strength (strong = low value) you get 5v (pullup) or 0v (pulldown). If you connect a variable resistor (e.g. a thermistor) between the analog pin 5v, you will get 5v at low values (strong pullup) and 'float' at high values (weak pullup). So the resistor is about more than just linearization.
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